1. Field of the Invention
Semiconductor diffusion/oxidation processing and low pressure CVD of silicon dioxide (doped and undoped), silicon (doped and undoped), single- and poly-crystalline, silicon nitride, and other thin films (conductive, semiconductive, and insulating) on semiconductor wafer substrates. Low pressure CVD of silicon dioxide (doped and undoped), polysilicon (doped and undoped), silicon nitride, and other thin films (conductive, semiconductive, and insulating) or larger for display panel substrates.
2. Description of the Prior Art
Chemical vapor deposition in existing furnace-tube CVD technology is based on a fundamental misunderstanding of the physical and chemical processes which occur in the reaction chamber. In Hochberg and in Zaferes, it is claimed that the cage structure creates a "turbulence" in the flow of reactant gas to achieve improved film uniformity, while in McMullen and in Dozier, it is claimed that the cage structure causes the gases to be "directed" around the substrates to achieve film uniformity. In fact, the cage provides a "depletion surface" which consumes most of the reactant before it can reach the substrates. This increases the mean free path of the reactant which increases film deposition uniformity and decreases film growth rate.
Two companies, the Thermco Division of Silicon Valley Group and BTU Engineering, dominate the furnace tube CVD process market at this time. They are continuing to try to push the performance of their existing designs beyond practical limits.
The diffusion/oxidation technology marketed by competitors is decades old. No significant research has been pursued in this field for a very long time. Rapid Thermal (single-wafer) processes are being developed, but there are significant technical and throughput hurdles to be overcome.
In CVD, additional non-furnace CVD suppliers are Novellus Systems and Applied Materials. These two companies market high-end, very expensive (&gt;$700,000) systems for silicon dioxide (LTO, PSG & BPSG) and passivation silicon nitride deposition. In spite of their very high price tags, the Novellus and Applied Materials technologies have developed considerable momentum in the marketplace. The source of this momentum lies in the failure of the furnace vendors to develop adequate process technology for their systems. Clearly, a furnace-based process which produces films of equal quality at one-third the capital cost would be far more economically attractive. Equal quality means equal physical/electrical performance (film thickness & compositional uniformity), equal process yield (particulate density and process-related device damage), and equal throughput. Currently available equipment has difficulty achieving within-wafer and wafer-to-wafer process uniformity better than .+-.10%. The primary problem with this process being too much doping at the wafer edge and not enough at wafer center. Existing systems are run with one-half to one-quarter size loads to improve batch yields. Process throughput suffers accordingly, and the result is a poor return on the investment in process hardware. The present invention hardware design will make possible doping and thickness uniformity of better than .+-.2% on full loads and on all wafer sizes from three inch to eight inch.
The present invention is a unique process hardware configuration for installation in existing or new horizontal or vertical furnace systems.
The present invention addresses a major weakness in existing furnace-based process hardware design. Currently available hardware does not "scale" easily to increases in wafer diameter. As size increases it becomes expotentially more difficult to adapt existing process technology to achieve previous standards of film quality. To make matters worse, film quality standards become more stringent each year. Existing systems produce acceptable results on three-, four- and five-inch wafers, but six-inch and larger wafers have proven to be a formidable obstacle. Delays in overcoming this obstacle have led to the success of Novellus Systems and Applied Materials in the single-wafer type PECVD systems. The new PECVD systems have one serious weakness, and that is expense. At three-quarters of a million dollars each, they are almost ten times the cost of existing furnace-based systems.
The present invention is developing the technology that will make available low-cost alternatives to the new systems while maintaining high standards of process quality. The equipment will be easier to automate than existing furnace-based systems and will adapt easily to the new vertical furnaces. It will consume less process gas than existing systems and will also generate less process-related particulate. This means that it will be able to run longer between clean cycles, and this, coupled with higher film growth rates, will result in more wafers being processed in less time (higher throughput). Most importantly, film thickness uniformity on large diameter wafers will be comparable to that achieved in the new single-wafer machines at a fraction of the capital cost.